1. Field of the Invention
The present invention relates generally to the assembly by welding, particularly by friction stir welding (FSW), of aluminum alloy parts that can be hardened by heat treatment and particularly alloys hardened by a thermo-mechanical treatment comprising a solution heat-treatment, quenching, cold working and aging.
2. Description of Related Art
In the fabrication of metal structures, particularly metal structures intended for aeronautic construction, welding can replace more conventional assembly techniques such as riveting. Welding methods allow parts to be assembled rapidly without creating holes, the latter frequently being crack initiation sites. Welding methods are known in which a melting of the materials takes place, such that, notably, oxyacetylene welding with filler metal, arc welding with non-consumable electrodes (TIG), arc welding with consumable electrodes or semi-automatic welding (MIG), laser welding, plasma welding, and electron-beam welding. Solid phase welding methods are also known such as friction stir welding or friction welding. Unfortunately, a certain number of problems have limited the use of welding, notably in the field of aeronautic construction.
An initial problem generally encountered with welded parts is the decrease in the mechanical strength of the welded joint in relation to the metal away from the welded zone. An elastic limit level is typically obtained in the welded zone equal to approximately 60% of the elastic limit of the metal used.
A second problem generally encountered is the increase in corrosion sensitivity of the welded zone.
Yet another problem encountered is the microstructure of the welded zone, for example the abnormal enlargement of gains during the solution treatment after welding. Protection of the welded zone was proposed to solve the corrosion problem. Patent application US 2008/047222 (Lockheed Martin) details cold vapor deposition on the FSW welded joint of a material to inhibit corrosion. Patent application WO 2004/014593 (Eclipse Aviation) details the application of a layer of sealing material, such as a sealing/monomer adhesive material, on the surfaces to be assembled so as to form a polymer sealing joint in situ owing to the welding heat in order to protect the element from weld corrosion. However, with this type of protection method, the welded joint remains sensitive to corrosion and a coating defect is always possible, particularly for assemblies of complex shape.
Various pre-welding or post-welding heat treatment sequences were proposed to improve the final properties of the welded joint or the welded assembly. Patent EP 1625242 (Alcan Rhenalu) details a method in which the elements to be welded undergo heat treatment prior to welding, at a temperature T lasting at least 2t1, t1 being defined as the minimum duration of a treatment at temperature T leading to a specific energy of the fusion peak, defined by AED, of less than 1 J/g. Patent EP 1799391 (Alcan Rhenalu) details a method in which one of the elements of the welded assembly is pre-aged prior to welding so as to obtain a structural element comprising at least two aluminum alloy parts which have different property trade-offs. U.S. Pat. No. 6,168,067 (McDonnell Douglas) details a method wherein the weld is performed on parts which have undergone solution heat-treating and hardening and are in an incomplete metallurgical temper and an additional precipitation heat treatment is performed after welding. Patent EP 1334793 (Boeing) details a friction stir welding method in which the welding tool is heated before the weld joint is welded and a solution heat-treatment and quenching operation is performed on the welded assembly. Patent application US2007/0138239 (Sumitomo Light Metals) teaches a method of joining by friction stir welding wherein members are joined in T4 temper and wherein a reversion treatment is carried out prior to or after the joining step. It should be noted that the time period for reversion treatment is not longer than 300 s. Patent application FR 2 868 084 A1 (Pechiney Rhenalu) teaches a method comprising the steps of a) solution heat treating an intermediate product b) optionally stretching c) aging. A main aspect of the invention according to this application is to carry out aging in a furnace with a controlled temperature profile. This application teaches applying the claimed method to a welded assembly, which results in solution heat treating and aging of the welded assembly or on the members before welding. Furthermore, the introduction of compression stresses on the surface of the welded joint is known, by techniques such as burnishing and shot peening to improve its fatigue and corrosion resistance properties.
Patent applications WO 03/082512 or US2005/0224562 (Surface Technology) propose a method intended for the assembly of metals such as aluminum, titanium or steel, which uses a welding device having a welding tool and a compression tool enabling a superficial residual compressive stress layer to be induced, during the last step of the process, along the surface of the weld line and to apply a specified quantity of cold working and superficial quenching in all thermomarked regions. In this last document, the specific problem of adding a welding operation within the production process of aluminum alloys hardened by a thermo-mechanical treatment comprising the successive steps of solution heat-treatment, quenching, cold working and aging is not foreseen.
Thus, for certain aluminum alloys, notably the 2XXX series, the optimum properties are obtained after thermo-mechanical treatment of this type. The metallurgical temper obtained is a temper defined as T8 in standard EN 515. This designation applies to products that are solution heat-treated, cold-worked and artificially aged which, after solution heat-treatment, are cold worked to improve strength, or in which the effect of cold work in flattening or straightening is recognized in mechanical property limits. Among the alloys for which temper T8 is particularly favorable, several alloys of the 2XXX series are noted and particularly the aluminum copper lithium alloys.
Patent application EP 0 227 563 A1 (CEGEDUR) teaches a specific under-aging treatment resulting in desensitization to exfoliating corrosion for aluminum alloys containing lithium.
The assembly method of these products by welding poses specific problems as it must be established at which stage the welding step is to be performed in relation to the various steps of the solution heat-treatment, quenching, work hardening and aging process.
The welding operation can, for example, be performed on parts in temper T3 or temper T8, as described in U.S. Pat. No. 5,259,897 (Martin Marietta). While the results obtained are generally relatively satisfactory in terms of mechanical strength or corrosion resistance, it would be useful for certain applications to further improve the performance characteristics obtained for the welded joint, notably in terms of mechanical strength and corrosion resistance.